8. Resources

8.1 Introduction

Java’s standard java.net.URL class and standard handlers for various URL prefixes
unfortunately are not quite adequate enough for all access to low-level resources. For
example, there is no standardized URL implementation that may be used to access a
resource that needs to be obtained from the classpath, or relative to a
ServletContext. While it is possible to register new handlers for specialized URL
prefixes (similar to existing handlers for prefixes such as http:), this is generally
quite complicated, and the URL interface still lacks some desirable functionality,
such as a method to check for the existence of the resource being pointed to.

8.2 The Resource interface

Spring’s Resource interface is meant to be a more capable interface for abstracting
access to low-level resources.

getInputStream(): locates and opens the resource, returning an InputStream for
reading from the resource. It is expected that each invocation returns a fresh
InputStream. It is the responsibility of the caller to close the stream.

isOpen(): returns a boolean indicating whether this resource represents a handle
with an open stream. If true, the InputStream cannot be read multiple times, and
must be read once only and then closed to avoid resource leaks. Will be false for
all usual resource implementations, with the exception of InputStreamResource.

getDescription(): returns a description for this resource, to be used for error
output when working with the resource. This is often the fully qualified file name or
the actual URL of the resource.

Other methods allow you to obtain an actual URL or File object representing the
resource (if the underlying implementation is compatible, and supports that
functionality).

The Resource abstraction is used extensively in Spring itself, as an argument type in
many method signatures when a resource is needed. Other methods in some Spring APIs
(such as the constructors to various ApplicationContext implementations), take a
String which in unadorned or simple form is used to create a Resource appropriate to
that context implementation, or via special prefixes on the String path, allow the
caller to specify that a specific Resource implementation must be created and used.

While the Resource interface is used a lot with Spring and by Spring, it’s actually
very useful to use as a general utility class by itself in your own code, for access to
resources, even when your code doesn’t know or care about any other parts of Spring.
While this couples your code to Spring, it really only couples it to this small set of
utility classes, which are serving as a more capable replacement for URL, and can be
considered equivalent to any other library you would use for this purpose.

It is important to note that the Resource abstraction does not replace functionality:
it wraps it where possible. For example, a UrlResource wraps a URL, and uses the
wrapped URL to do its work.

8.3 Built-in Resource implementations

There are a number of Resource implementations that come supplied straight out of the
box in Spring:

8.3.1 UrlResource

The UrlResource wraps a java.net.URL, and may be used to access any object that is
normally accessible via a URL, such as files, an HTTP target, an FTP target, etc. All
URLs have a standardized String representation, such that appropriate standardized
prefixes are used to indicate one URL type from another. This includes file: for
accessing filesystem paths, http: for accessing resources via the HTTP protocol,
ftp: for accessing resources via FTP, etc.

A UrlResource is created by Java code explicitly using the UrlResource constructor,
but will often be created implicitly when you call an API method which takes a String
argument which is meant to represent a path. For the latter case, a JavaBeans
PropertyEditor will ultimately decide which type of Resource to create. If the path
string contains a few well-known (to it, that is) prefixes such as classpath:, it will
create an appropriate specialized Resource for that prefix. However, if it doesn’t
recognize the prefix, it will assume the this is just a standard URL string, and will
create a UrlResource.

8.3.2 ClassPathResource

This class represents a resource which should be obtained from the classpath. This uses
either the thread context class loader, a given class loader, or a given class for
loading resources.

This Resource implementation supports resolution as java.io.File if the class path
resource resides in the file system, but not for classpath resources which reside in a
jar and have not been expanded (by the servlet engine, or whatever the environment is)
to the filesystem. To address this the various Resource implementations always support
resolution as a java.net.URL.

A ClassPathResource is created by Java code explicitly using the ClassPathResource
constructor, but will often be created implicitly when you call an API method which
takes a String argument which is meant to represent a path. For the latter case, a
JavaBeans PropertyEditor will recognize the special prefix classpath: on the string
path, and create a ClassPathResource in that case.

8.3.3 FileSystemResource

This is a Resource implementation for java.io.File handles. It obviously supports
resolution as a File, and as a URL.

8.3.4 ServletContextResource

This is a Resource implementation for ServletContext resources, interpreting
relative paths within the relevant web application’s root directory.

This always supports stream access and URL access, but only allows java.io.File access
when the web application archive is expanded and the resource is physically on the
filesystem. Whether or not it’s expanded and on the filesystem like this, or accessed
directly from the JAR or somewhere else like a DB (it’s conceivable) is actually
dependent on the Servlet container.

8.3.5 InputStreamResource

A Resource implementation for a given InputStream. This should only be used if no
specific Resource implementation is applicable. In particular, prefer
ByteArrayResource or any of the file-based Resource implementations where possible.

In contrast to other Resource implementations, this is a descriptor for an already
opened resource - therefore returning true from isOpen(). Do not use it if you need
to keep the resource descriptor somewhere, or if you need to read a stream multiple
times.

8.3.6 ByteArrayResource

This is a Resource implementation for a given byte array. It creates a
ByteArrayInputStream for the given byte array.

It’s useful for loading content from any given byte array, without having to resort to a
single-use InputStreamResource.

8.4 The ResourceLoader

The ResourceLoader interface is meant to be implemented by objects that can return
(i.e. load) Resource instances.

All application contexts implement the ResourceLoader interface, and therefore all
application contexts may be used to obtain Resource instances.

When you call getResource() on a specific application context, and the location path
specified doesn’t have a specific prefix, you will get back a Resource type that is
appropriate to that particular application context. For example, assume the following
snippet of code was executed against a ClassPathXmlApplicationContext instance:

What would be returned would be a ClassPathResource; if the same method was executed
against a FileSystemXmlApplicationContext instance, you’d get back a
FileSystemResource. For a WebApplicationContext, you’d get back a
ServletContextResource, and so on.

As such, you can load resources in a fashion appropriate to the particular application
context.

On the other hand, you may also force ClassPathResource to be used, regardless of the
application context type, by specifying the special classpath: prefix:

When a class implements ResourceLoaderAware and is deployed into an application
context (as a Spring-managed bean), it is recognized as ResourceLoaderAware by the
application context. The application context will then invoke the
setResourceLoader(ResourceLoader), supplying itself as the argument (remember, all
application contexts in Spring implement the ResourceLoader interface).

Of course, since an ApplicationContext is a ResourceLoader, the bean could also
implement the ApplicationContextAware interface and use the supplied application
context directly to load resources, but in general, it’s better to use the specialized
ResourceLoader interface if that’s all that’s needed. The code would just be coupled
to the resource loading interface, which can be considered a utility interface, and not
the whole Spring ApplicationContext interface.

As of Spring 2.5, you can rely upon autowiring of the ResourceLoader as an alternative
to implementing the ResourceLoaderAware interface. The "traditional" constructor and
byType autowiring modes (as described in Section 7.4.5, “Autowiring collaborators”) are now capable
of providing a dependency of type ResourceLoader for either a constructor argument or
setter method parameter respectively. For more flexibility (including the ability to
autowire fields and multiple parameter methods), consider using the new annotation-based
autowiring features. In that case, the ResourceLoader will be autowired into a field,
constructor argument, or method parameter that is expecting the ResourceLoader type as
long as the field, constructor, or method in question carries the @Autowired
annotation. For more information, see Section 7.9.2, “@Autowired”.

8.6 Resources as dependencies

If the bean itself is going to determine and supply the resource path through some sort
of dynamic process, it probably makes sense for the bean to use the ResourceLoader
interface to load resources. Consider as an example the loading of a template of some
sort, where the specific resource that is needed depends on the role of the user. If the
resources are static, it makes sense to eliminate the use of the ResourceLoader
interface completely, and just have the bean expose the Resource properties it needs,
and expect that they will be injected into it.

What makes it trivial to then inject these properties, is that all application contexts
register and use a special JavaBeans PropertyEditor which can convert String paths
to Resource objects. So if myBean has a template property of type Resource, it can
be configured with a simple string for that resource, as follows:

Note that the resource path has no prefix, so because the application context itself is
going to be used as the ResourceLoader, the resource itself will be loaded via a
ClassPathResource, FileSystemResource, or ServletContextResource (as appropriate)
depending on the exact type of the context.

If there is a need to force a specific Resource type to be used, then a prefix may be
used. The following two examples show how to force a ClassPathResource and a
UrlResource (the latter being used to access a filesystem file).

8.7 Application contexts and Resource paths

8.7.1 Constructing application contexts

An application context constructor (for a specific application context type) generally
takes a string or array of strings as the location path(s) of the resource(s) such as
XML files that make up the definition of the context.

When such a location path doesn’t have a prefix, the specific Resource type built from
that path and used to load the bean definitions, depends on and is appropriate to the
specific application context. For example, if you create a
ClassPathXmlApplicationContext as follows:

The bean definition will be loaded from a filesystem location, in this case relative to
the current working directory.

Note that the use of the special classpath prefix or a standard URL prefix on the
location path will override the default type of Resource created to load the
definition. So this FileSystemXmlApplicationContext…​

will actually load its bean definitions from the classpath. However, it is still a
FileSystemXmlApplicationContext. If it is subsequently used as a ResourceLoader, any
unprefixed paths will still be treated as filesystem paths.

Constructing ClassPathXmlApplicationContext instances - shortcuts

The ClassPathXmlApplicationContext exposes a number of constructors to enable
convenient instantiation. The basic idea is that one supplies merely a string array
containing just the filenames of the XML files themselves (without the leading path
information), and one also supplies a Class; the ClassPathXmlApplicationContext
will derive the path information from the supplied class.

An example will hopefully make this clear. Consider a directory layout that looks like
this:

com/
foo/
services.xml
daos.xml
MessengerService.class

A ClassPathXmlApplicationContext instance composed of the beans defined in the
'services.xml' and 'daos.xml' could be instantiated like so…​

Please do consult the ClassPathXmlApplicationContext javadocs for details
on the various constructors.

8.7.2 Wildcards in application context constructor resource paths

The resource paths in application context constructor values may be a simple path (as
shown above) which has a one-to-one mapping to a target Resource, or alternately may
contain the special "classpath*:" prefix and/or internal Ant-style regular expressions
(matched using Spring’s PathMatcher utility). Both of the latter are effectively
wildcards

One use for this mechanism is when doing component-style application assembly. All
components can 'publish' context definition fragments to a well-known location path, and
when the final application context is created using the same path prefixed via
classpath*:, all component fragments will be picked up automatically.

Note that this wildcarding is specific to use of resource paths in application context
constructors (or when using the PathMatcher utility class hierarchy directly), and is
resolved at construction time. It has nothing to do with the Resource type itself.
It’s not possible to use the classpath*: prefix to construct an actual Resource, as
a resource points to just one resource at a time.

The resolver follows a more complex but defined procedure to try to resolve the
wildcard. It produces a Resource for the path up to the last non-wildcard segment and
obtains a URL from it. If this URL is not a jar: URL or container-specific variant
(e.g. zip: in WebLogic, wsjar in WebSphere, etc.), then a java.io.File is
obtained from it and used to resolve the wildcard by traversing the filesystem. In the
case of a jar URL, the resolver either gets a java.net.JarURLConnection from it or
manually parses the jar URL and then traverses the contents of the jar file to resolve
the wildcards.

Implications on portability

If the specified path is already a file URL (either explicitly, or implicitly because
the base ResourceLoader is a filesystem one, then wildcarding is guaranteed to work in
a completely portable fashion.

If the specified path is a classpath location, then the resolver must obtain the last
non-wildcard path segment URL via a Classloader.getResource() call. Since this is just
a node of the path (not the file at the end) it is actually undefined (in the
ClassLoader javadocs) exactly what sort of a URL is returned in this case. In
practice, it is always a java.io.File representing the directory, where the classpath
resource resolves to a filesystem location, or a jar URL of some sort, where the
classpath resource resolves to a jar location. Still, there is a portability concern on
this operation.

If a jar URL is obtained for the last non-wildcard segment, the resolver must be able to
get a java.net.JarURLConnection from it, or manually parse the jar URL, to be able to
walk the contents of the jar, and resolve the wildcard. This will work in most
environments, but will fail in others, and it is strongly recommended that the wildcard
resolution of resources coming from jars be thoroughly tested in your specific
environment before you rely on it.

The Classpath*: portability classpath*: prefix

When constructing an XML-based application context, a location string may use the
special classpath*: prefix:

This special prefix specifies that all classpath resources that match the given name
must be obtained (internally, this essentially happens via a
ClassLoader.getResources(…​) call), and then merged to form the final application
context definition.

Note

The wildcard classpath relies on the getResources() method of the underlying
classloader. As most application servers nowadays supply their own classloader
implementation, the behavior might differ especially when dealing with jar files. A
simple test to check if classpath* works is to use the classloader to load a file from
within a jar on the classpath:
getClass().getClassLoader().getResources("<someFileInsideTheJar>"). Try this test with
files that have the same name but are placed inside two different locations. In case an
inappropriate result is returned, check the application server documentation for
settings that might affect the classloader behavior.

The classpath*: prefix can also be combined with a PathMatcher pattern in the
rest of the location path, for example classpath*:META-INF/*-beans.xml. In this
case, the resolution strategy is fairly simple: a ClassLoader.getResources() call is
used on the last non-wildcard path segment to get all the matching resources in the
class loader hierarchy, and then off each resource the same PathMatcher resolution
strategy described above is used for the wildcard subpath.

Other notes relating to wildcards

Please note that classpath*: when combined with Ant-style patterns will only work
reliably with at least one root directory before the pattern starts, unless the actual
target files reside in the file system. This means that a pattern like
classpath*:*.xml will not retrieve files from the root of jar files but rather only
from the root of expanded directories. This originates from a limitation in the JDK’s
ClassLoader.getResources() method which only returns file system locations for a
passed-in empty string (indicating potential roots to search).

Ant-style patterns with classpath: resources are not guaranteed to find matching
resources if the root package to search is available in multiple class path locations.
This is because a resource such as

com/mycompany/package1/service-context.xml

may be in only one location, but when a path such as

classpath:com/mycompany/**/service-context.xml

is used to try to resolve it, the resolver will work off the (first) URL returned by
getResource("com/mycompany");. If this base package node exists in multiple
classloader locations, the actual end resource may not be underneath. Therefore,
preferably, use " `classpath*:`" with the same Ant-style pattern in such a case, which
will search all class path locations that contain the root package.

8.7.3 FileSystemResource caveats

A FileSystemResource that is not attached to a FileSystemApplicationContext (that
is, a FileSystemApplicationContext is not the actual ResourceLoader) will treat
absolute vs. relative paths as you would expect. Relative paths are relative to the
current working directory, while absolute paths are relative to the root of the
filesystem.

For backwards compatibility (historical) reasons however, this changes when the
FileSystemApplicationContext is the ResourceLoader. The
FileSystemApplicationContext simply forces all attached FileSystemResource instances
to treat all location paths as relative, whether they start with a leading slash or not.
In practice, this means the following are equivalent:

In practice, if true absolute filesystem paths are needed, it is better to forgo the use
of absolute paths with FileSystemResource / FileSystemXmlApplicationContext, and
just force the use of a UrlResource, by using the file: URL prefix.